Mark xhr.status=0 as successful

File protocol yields status code 0.
To support reveal on mobile application created using Phonegap/Cordova and for local debugging- xhr.status=0 should be marked as successful.

jQuery uses the same concept:
http://code.jquery.com/jquery-2.0.1.js

```javascript
xhrSuccessStatus = {
        // file protocol always yields status code 0, assume 200
        0: 200,
```